Wi-Fi 6 (802.11ax) Fundamentals: What is MU-MIMO?

The Institute of Electrical and Electronics Engineers (IEEE) has ratified five major iterations of the 802.11 Wi-Fi protocol, culminating with Wi-Fi 5 (802.11ac) in 2013. However, despite a significant increase in speed, many organizations still find themselves limited by the Wi-Fi 5 standard, particularly in high-density venues such as stadiums, convention centers, transportation hubs and auditoriums. To meet the challenges of high-density deployments, the IEEE recently introduced the Wi-Fi 6 (802.11ax) standard – which is the first to bridge the performance gap towards 10 gigabit speeds. With an expected four-fold capacity increase over its Wi-Fi 5 (802.11ac) predecessor, Wi-Fi 6 is successfully transitioning Wi-Fi from a best-effort endeavor to a deterministic wireless technology that is fast becoming the de-facto medium for internet connectivity.

Indeed, Wi-Fi 6 (802.11ax) deployed in dense device environments supports higher service-level agreements (SLAs) with more concurrently connected users and devices and more diverse usage profiles. This is made possible by a range of technologies that optimize spectral efficiency, increase throughput and reduce power consumption. These include Multi-User Multiple Input Multiple Output (MU-MIMO), Target Wake Time (TWT), Orthogonal Frequency-Division Multiple Access (OFDMA), BSS Coloring and 1024-QAM. In this article, we’ll be taking a closer look at how the Wi-Fi 6 (802.11ax) MU-MIMO mechanism addresses the challenges of dense device environments by adding uplink support for simultaneous (upstream and downstream) client data transmissions.

MU-MIMO describes a set of multiple-input and multiple-output (MIMO) technologies for wireless communication. MU-MIMO was first introduced to the wireless world in 2015 as part of the Wi-Fi 5 (802.11ac) standard, with the Wi-Fi 6 (802.11ax) protocol adding MU-MIMO support for uplink. MU-MIMO can be used in networks where a single access point (AP) must communicate with multiple clients simultaneously to improve overall efficiency.

MU-MIMO (Wi-Fi 5/802.11ac)

MU-MIMO allows an access
point to communicate with multiple devices simultaneously. It is part of
the Wi-Fi 5 (802.11ac) Wave 2 standard. With MU-MIMO, an access point or a
wireless router can communicate with multiple network clients at the same time
– thus increasing the speed of the data transfer without congestion. A significant
advantage of MU-MIMO is its support for transmitting data from an AP to devices
in a downlink connection. In addition, MU-MIMO reduces the delay for each
end-device receiving the data and enhances the device connectivity between
multiple end users.

However, MU-MIMO also has certain
limitations. Its functionality works only within the 5GHz band, as Wi-Fi 5 (802.11ac)
is defined only in this band. In addition, MU-MIMO works only when transmitting
data from an AP to a client in downlink transmissions – and does not operate in
reverse. Moreover, MU-MIMO supports only a limited number of simultaneous data streams.

MU-MIMO (Wi-Fi 6 / 802.11ax)

Wi-Fi 6 (802.11ax) leverages
the multi-user version of OFDMA and MU-MIMO for better efficiency of uplink and
downlink transmissions. OFDMA allows the transmission of big chunks of data
over a single noisy channel. This technique works by splitting a single signal
into multiple smaller signals that are transmitted. The combination of OFDMA
and MU-MIMO allows Wi-Fi 6 (802.11ax) to achieve increased capacity, improved
coverage and performance in ultra-high-density environments.

UL MU-MIMO is a new key feature
introduced with Wi-Fi 6 (802.11ax). By leveraging UL MU-MIMO, multiple clients connected
to the access point will be able to send acknowledgement responses (ack)
simultaneously, thus saving airtime. This ultimately improves network
throughput and efficiency.

Another important Wi-Fi 6 (802.11ax) feature is its support for 20MHz-only clients. This is particularly beneficial for low-cost IoT devices that require low power and pack very small batteries. In contrast, Wi-Fi 5 (802.11ac) mandates 80MHz clients. The Wi-Fi 6 (802.11ax) protocol enables simultaneous upstream and downstream MU-MIMO data transmissions on the same frequency. This results in higher Wi-Fi performance, especially in higher-density environments such as stadiums, convention centers, transportation hubs and auditoriums.

It should be noted that MU-MIMO and OFDMA provide complementary techniques to concurrently serve multiple users. More specifically, MU-MIMO is most effective at close to mid-range, whereas OFDMA is effective at all ranges, close, medium and far. Moreover, MU-MIMO best serves multiple user with full buffer traffic, while OFDMA is utilized when multiple connections transmit relatively limited amounts of data.

Conclusion

Christian Kim, Senior Analyst IoT, Connectivity and Telecom
Electronics at IHS Markit, estimates that total Wi-Fi 6 (802.11ax) device shipments will increase
to 58 million units in 2021. Meanwhile, IDC
sees Wi-Fi 6 (802.11ax)
deployments ramping significantly in 2019 and becoming the
dominant enterprise Wi-Fi standard by 2021. This is because the new Wi-Fi 6 (802.11ax)
standard offers up to a four-fold capacity increase over its Wi-Fi 5 (802.11ac)
Wave 2 predecessor.

With Wi-Fi 6 (802.11ax), multiple APs deployed in dense device environments can
collectively deliver required quality-of-service (QoS) to more clients with
more diverse usage profiles. This is made possible by a range of technologies,
such as the Wi-Fi 6 (802.11ax) iteration of MU-MIMO, which enables simultaneous
MU-MIMO data transmissions on the same frequency. From our perspective, Wi-Fi 6
(802.11ax) is playing a critical role in helping Wi-Fi evolve in to a collision-free,
deterministic wireless technology that dramatically increases aggregate network
throughput to address high-density venues and beyond. Last, but certainly not
least, Wi-Fi 6 (802.11ax) access points are also expected to enhance the
overall Wi-Fi experience by providing tangible performance benefits for legacy
wireless devices.